There are condenser microphone units which use diaphragms at rear acoustic terminals (for example, see Patent Literature 1: Japanese Patent Application Publication No. 2005-184347). One example of such condenser microphone units is shown in FIG. 4 and will be described below.
This condenser microphone unit 10 has a basic configuration including a first condenser element 10a and a second condenser element 10b which are coupled via a coupling ring 22 at the center.
The first condenser element 10a and the second condenser element 10b have the same configuration. Hereinafter, the first condenser element 10a will be mainly described, and reference numerals of the corresponding components of the second condenser element 10b will be described in brackets.
The condenser element 10a (10b) is configured by incorporating a diaphragm supporting member 12a (12b) over which a diaphragm 11a (11b) stretches with predetermined tension applied, a spacer ring 13a (13b), a fixed pole 14a (14b) and an insulation base 15a (15b) in this order inside a case 16a (16b) which is formed in a ring shape so as to have electric insulation.
The diaphragm supporting member 12a (12b) is formed with a disc body having an acoustic hole for taking in an acoustic wave into the diaphragm 11a (11b). At a substantially central portion of the diaphragm supporting member 12a (12b), an electrode lead-out terminal 121 is attached. Although FIG. 4 shows only one acoustic hole, a plurality of acoustic holes 141 are pierced at the fixed pole 14a (14b).
The insulation base 15a (15b) is formed in a plate shape with a standing peripheral portion so that an air chamber with a predetermined volume is formed between the insulation base 15a (15b) and the fixed pole 14a (14b) and through-holes 152 are pierced at respective centers of the bottom portions. Further, an external thread 151 is formed at an outer periphery surface of the insulation base 15a (15b).
The first condenser element 10a and the second condenser element 10b are coupled to each other by the external threads 151, 151 being screwed into internal threads 221 of the coupling ring 22 in a state where the insulation bases 15a, 15b are placed back-to-back. At this time, an acoustic resistance material 31 which has a larger diameter than those of the through-holes 152 is arranged coaxially with respect to the through-holes 152 between the insulation base 15a and the insulation base 15b. That is, the through-holes 152 of the insulation bases 15a, 15b are acoustically connected by the acoustic resistance material 31. It should be noted that a gasket 32 for preventing sound leakage is disposed around the acoustic resistance material 31.
This configuration allows an amount of compression of the acoustic resistance material 31, that is, an acoustic resistance value to be variable according to screw amounts of the insulation bases 15a, 15b with respect to the coupling ring 22. Accordingly, it is possible to adjust the acoustic resistance value between the diaphragm 11a and the diaphragm 11b in a state where the first condenser element 10a and the second condenser element 10b are assembled.
In this condenser microphone unit 10, the condenser elements 10a and 10b are both unidirectional. Therefore, as described in, for example, Patent Literature 2 (Japanese Patent Application Publication No. H07-143595), it is possible to change directionality by changing a voltage to be applied to each of the condenser elements 10a, 10b. Further, if the diaphragm 11a at the side of the first condenser element 10a is used as a front acoustic terminal, the diaphragm 11b at the side of the second condenser element 10b acts as a rear acoustic terminal.
In the above-described condenser microphone unit 10, the electrode lead-out terminals 121 are attached to the diaphragm supporting members 12a, 12b of both the first condenser element 10a and the second condenser element 10b so as to obtain a sound signal. Therefore, because it is difficult to directly apply such a condenser microphone unit to a handheld (mobile) condenser microphone, conventionally, the condenser microphone unit has been only applied to a side-entry type condenser microphone.
Therefore, a problem to be solved by the present invention is to enable application of a condenser microphone unit having two unidirectional condenser elements as described above to a handheld condenser microphone.